//解数独
//测试链接：https://leetcode.cn/problems/sudoku-solver/
public class SolveSudoku {
    boolean[][] row, col;
    boolean[][][] grid;

    public void solveSudoku(char[][] board) {
        row = new boolean[9][10];
        col = new boolean[9][10];
        grid = new boolean[3][3][10];
        // 初始化
        for (int i = 0; i < 9; i++)
            for (int j = 0; j < 9; j++)
                if (board[i][j] != '.') {
                    int num = board[i][j] - '0';
                    row[i][num] = col[j][num] = grid[i / 3][j / 3][num] = true;
                }
        dfs(board);
    }

    public boolean dfs(char[][] board) {
        for (int i = 0; i < 9; i++) {
            for (int j = 0; j < 9; j++) {
                if (board[i][j] == '.') {
                    // 填数
                    for (int num = 1; num <= 9; num++) {
                        // 剪枝
                        if (!row[i][num] && !col[j][num] && !grid[i / 3][j / 3][num]) {
                            board[i][j] = (char) ('0' + num);
                            row[i][num] = col[j][num] = grid[i / 3][j / 3][num] = true;
                            if (dfs(board) == true)
                                return true; // 重点理解
                            // 恢复现场
                            board[i][j] = '.';
                            row[i][num] = col[j][num] = grid[i / 3][j / 3][num] = false;
                        }
                    }
                    return false; // 重点理解
                }
            }
        }
        return true; // 重点理解
    }
}
